1. Field of the Invention
The present invention relates generally to boroaluminosilicate glasses. More specifically, the invention relates to an alkali fluorine-doped boroaluminosilicate glasses which exhibit excellent UV transmission.
2. Technical Background
Glasses capable of transmitting well in the ultraviolet are rare, and those that do so are difficult to fabricate. For example, extraordinary UV transmission can be obtained in silica prepared by a chemical vapor deposition process (e.g., the direct process or various types of soot/preform processes), but such processes are very expensive with regard to raw materials (e.g., octamethylcyclotetrasilane or SiCl4). Additionally, other shortcomings of these fused silica manufacturing processes include low yields, as well as the difficulty in tailoring the process so as to be a continuous manufacturing process.
It is well known that certain fluoride glasses, e.g., beryllium fluoride, exhibit excellent UV transmission, however these glasses also have certain problems associated with their manufacture; specifically the toxicity of the elements involved, materials handling problems associated with fluorine, the need for controlled melting atmospheres, limited working ranges that interfere with precision casting, etc.
Fluoride crystals, like fused silica and fluoride glasses, exhibit excellent UV transmission, but also present very difficult manufacturing challenges; particularly, the difficulty in scaling up the process to produce large parts. In certain applications, such as LCD lithography, the need for deep UV transmission is less critical than the need for large, high quality parts, and thus crystalline materials, given this difficulty in scaling up the process to produce large parts, would be totally impractical for such applications.
Furthermore, it is often desirable to be able to take advantage of the refractive indices or dispersion characteristics of two or more optical materials to minimize chromatic aberration, vary focal length, etc. Many glasses are available for such combinations for visible applications, but as above, the number of materials with distinctive indices or suitable transmission in the UV is quite small. Were such materials required for large-scale applications, the only practical material would be high-purity fused silica, and therefore color correction and so on is simply not an option using conventional optical approaches.
As such there is a need in the lithography industry for a glass or suite of glasses with the desirable UV transmission of, for example, fluoride glasses, but with the ease of melting and forming of conventional oxide glasses. These glasses would necessarily be made using high purity materials and high-purity melting methods, and so a ready supply of comparatively inexpensive raw materials and a relatively inexpensive melting method would be highly desirable. The instant invention is directed at glasses which fulfill this need.